1. Field of the Invention
The present invention relates to an ultrasonic measuring method for obtaining physical characteristics of the inside of a subject of measurement by transmitting an ultrasonic wave into the subject, receiving a reflected ultrasonic wave from the inside of the subject and processing the received signal.
2. Description of the Prior Art
Ultrasonic measuring technology has made rapid progress and its application, too, is now broadening from metal flaw detection to medical diagnosis. For such ultrasonic measuring technology an ultrasonic reflection method is employed. The ultrasonic reflection method utilizes the reflection of an ultrasonic wave from the inside of an object at an acoustically heterogeneous point therein, and receives the reflected ultrasonic wave, producing an echogram. The reflected ultrasonic wave is obtained based on the state of the acoustically heterogeneous point, including the influences (attenuation, reflection) on the ultrasonic wave by that portion of the object through which it propagates to the acoustically heterogeneous point. The magnitude of the reflected ultrasonic wave depends largely upon the angle of incidence of the ultrasonic wave to the acoustically heterogeneous point and the state of the surface of the point, and does not exactly indicate physical characteristics of the object, such as an acoustic impedance and so forth.
For this reason, the existing ultrasonic reflection method is used mainly for the indication of the position of the acoustically heterogeneous point, and it cannot be utilized for measuring the physical characteristics of an object (for example, an attenuation characteristic).
Various attempts have been made to broaden the application of such an ultrasonic reflection method. These attempts are disclosed, for instance, in Japanese Patent Application Gazette No. 24798/77 published on July 4, 1977 and Japanese Patent "Kokai"(Laid-Open) Gazette No. 38490/74 laid open on Apr. 10, 1974. These prior art techniques are to measure an absortion coefficient of a substance by transmitting and receiving ultrasonic waves having different frequency components and lessening the influence of the reflections of the ultrasonic waves at a heterogeneous interface in the acoustic path through utilization of the sound pressure ratio of each frequency component to the others.
However, such prior art techniques are effective only when the received reflection intensity is sufficient. In general, in the case where the reflection intensity is insufficient, as the signal to noise ratio approaches unity, the denominator and numerator of the sound pressure intensity ratio may sometimes approach zero independently of each other due to the irregularity of the noise amplitude that will introduce dispersion in the value of the ratio between zero and infinity, resulting in a large error.